# year value 1901 19.54739 1902 19.31712 1903 19.24383 1904 19.24491 1905 19.35677 1906 19.49578 1907 19.1322 1908 19.32892 1909 19.3677 1910 19.41052 1911 19.31647 1912 19.13873 1913 19.25163 1914 19.43966 1915 19.47253 1916 19.27019 1917 19.2937 1918 19.21586 1919 19.37503 1920 19.34194 1921 19.49913 1922 19.38069 1923 19.3474 1924 19.41548 1925 19.36573 1926 19.47396 1927 19.45989 1928 19.38748 1929 19.26181 1930 19.57497 1931 19.70039 1932 19.5507 1933 19.43914 1934 19.51367 1935 19.52015 1936 19.62947 1937 19.66083 1938 19.63703 1939 19.67503 1940 19.65454 1941 19.71638 1942 19.5345 1943 19.43672 1944 19.65017 1945 19.61139 1946 19.45358 1947 19.54254 1948 19.6265 1949 19.42372 1950 19.29838 1951 19.49308 1952 19.57155 1953 19.63039 1954 19.4699 1955 19.4927 1956 19.16875 1957 19.48418 1958 19.50033 1959 19.59479 1960 19.52673 1961 19.54666 1962 19.43957 1963 19.52113 1964 19.29265 1965 19.31365 1966 19.60114 1967 19.35663 1968 19.27042 1969 19.51766 1970 19.55706 1971 19.33853 1972 19.43328 1973 19.69016 1974 19.38788 1975 19.52475 1976 19.2023 1977 19.61207 1978 19.27403 1979 19.56156 1980 19.6286 1981 19.70985 1982 19.52524 1983 19.77882 1984 19.57173 1985 19.51015 1986 19.59499 1987 19.82194 1988 19.95172 1989 19.72981 1990 19.87816 1991 19.96764 1992 19.3939 1993 19.68545 1994 19.89818 1995 20.10206 1996 19.79365 1997 19.99082 1998 20.46212 1999 19.97742 2000 19.98149 2001 20.16124 2002 20.23566 2003 20.19114 2004 19.92782 2005 20.32582 2006 20.33943 2007 20.17345 2008 20.16777 2009 20.19088 2010 20.43584 2011 20.34476 2012 20.41023 2013 20.29832 2014 20.3248 2015 20.49701 2016 20.60259 2017 20.49595 2018 20.36764